The present disclosure relates to formation of latent images in a radiation sensitive layer applied to a substrate that is transparent to or transmissive of radiation at the exposing wavelength. In particular, it relates to so-called backside lithography, in which the final lens of an exposing system is positioned to project electromagnetic radiation through a first side of the transparent substrate and expose a radiation sensitive layer that overlays a second side of the transparent substrate that is opposite the first side. Five alternative embodiments for further treatment to form a radiation opaque layer corresponding to the latent image (the image or its inverse) are described. These methods and corresponding devices are useful for producing masks (sometimes called reticles), for producing latent images in semiconductor devices and for forming features of semiconductor devices using masks.
In lithography, resolution is limited by the optical NA (Numerical Aperture) of the lithography system. One way to increase the NA, and the resolution, is to use a medium with higher refractive index between the final lens and the image substrate. This medium can for instance be an immersion liquid or solid.
FIG. 1 depicts how immersion lithography might be implemented in an optical photomask generator. A substrate (110) is overlaid by chrome (112) and photoresist (114). The substrate typically is quartz, which has limited transmission wavelengths of electromagnetic radiation shorter than 160 nm, unless prepared to be especially dry with fewer hydroxyl groups in the quartz than normal. Other substrates may be used, depending in part on their transparency to radiation used for patterning, such as Corning's ULE and HPFS glass. Chrome is typically used as an opaque or radiation stopping layer. Photoresist is exposed using the radiation and developed to produce a pattern. The photoresist is exposed to electromagnetic radiation focused by an optical path including a final lens (101). The optical path may include shaped mirrors that help focus radiation of a short wavelength. The final lens may be an interface between vacuum or other containment of a mirror-based optical path and the workpiece. An immersion liquid or solid (105) positioned in contact with both the final lens (101) and the resist layer (114) overlying the substrate is useful because it has a higher refractive index than an air gap. Conventional methods for developing and partially removing a resist layer are known in the art. Patterning the resist is part of forming devices on the substrate, including features of semiconductor devices.
An opportunity arises to develop alternative processes for forming latent images, for creating masks to be used in production of devices and, ultimately, to form features of devices on a device substrate. This may result in better, more easily configured and controlled, more resilient and transparent processes, tools and devices.